1. Field of the Invention
This invention is in the field of atomic force microscopy, specifically relating to atomic force microscopes (AFMs) operating with the probe in an oscillating mode and a method of improving the operation in an oscillating mode.
2. Description of the Related Art
Atomic Force Microscopes (AFMs) operate by scanning a probe over a surface using a high resolution three axis scanner, usually creating relative motion between the probe and the sample while measuring the topography or some other surface property, as described in Hansma et al. in U.S. Pat. No. RE34,489. AFMs typically include a probe, usually a very small lever fixed at one end with a sharp probe tip attached to the opposite, or free, end. The probe tip is brought very near to or into contact with a surface to be examined, and the deflection of the lever in response to the tip's interaction with the surface is measured with an extremely sensitive deflection detector, often an optical lever system such as described by Hansma et al, or some other deflection detector such as a strain gauge, capacitance sensor, or others well known in the art. Using piezoelectric scanners, optical lever deflection detectors, and very small probe lever arms fabricated using photolithographic techniques, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum. Because of their resolution and versatility, AFMs are important measurement devices in a diversity of fields ranging from semiconductor manufacturing to biological research.
Early AFMs operated in what is commonly referred to as contact mode by scanning the tip in contact with the surface, thereby causing the lever to bend in response to the sample features. Typically, the output of the deflection detector was used as an error signal to a feedback loop which servoed the vertical axis scanner up or down to maintain a constant preset lever deflection as the tip was scanned laterally over the sample surface topography. The servo signal versus lateral position created a topographic map (or image) of the sample surface. Thus, the AFM maintained a constant lever deflection and accordingly a constant force on the sample surface during lateral scanning. Using very small, microfabricated levers, the tip-sample force in contact mode AFMs could be maintained at force levels sufficiently small to allow imaging of biological substances, and in some cases achieve atomic resolution. In addition, an AFM can measure very small force interactions between the tip and sample, and by suitably preparing the probe, such as coating it with an appropriate material, other parameters such as magnetic or electric fields may be measured with an AFM.
However, for many samples, particularly in ambient atmospheric environment, the fluid layer found on all surfaces, along with other factors affecting the tip-sample interaction, may cause the probe tip to stick to the sample surface during contact mode scanning. This sticking may degrade the image quality or damage the sample surface or the probe tip. This effect may be reduced by immersing the probe and sample in liquid or vacuum, but such operation is not convenient for many applications.
Contact mode is basically a DC measurement. Other modes of operation have been developed in which the lever (or another probe configuration) is oscillated. At this time, three oscillating probe modes are common in AFMs, non-contact (Martin, et al, J. Applied Physics 61(10) May 15, 1987), shear force (U.S. Pat. No. 5,254,854 by Betzig et al), and Tapping.TM. or TappingMode.TM.. (Tapping and TappingMode are trademarks of Digital Instruments, Inc.). U.S. patents relating to Tapping and TappingMode include U.S. Pat. Nos. 5,266,801, 5,412,980 and 5,519,212, by Elings et al., all of which are hereby incorporated by reference. These modes, particularly TappingMode, have become extremely commercially successful. This invention is directed at improving the operation of TappingMode AFM, but may also be directed to other oscillating AFM modes of operation.
In oscillating AFM operation, the probe is oscillated, typically at the probe's resonant frequency, and brought near the sample surface. In all oscillation modes, the probe is first oscillated at a distance sufficient from the sample to prevent direct probe-sample interaction, referred to as the free oscillation. The effect of the surface on the free oscillation parameters of the probe is used as the signal of interest, either as an error signal for a feedback loop or as direct measure of tip-sample force interaction. For example, many AFMs employ a feedback loop that uses changes in the oscillation amplitude due to interaction with the surface to maintain that amplitude substantially constant. In non-contact and TappingMode operations, the free oscillation is substantially perpendicular to the plane of the sample surface, while in shear force measurement operation, the free oscillation is essentially parallel with the plane of the sample surface. Non-contact operation relies on non-contact force gradients to affect the resonant properties of the probe in a measurable fashion, while TappingMode operation relies on the far more robust interaction of actually striking the surface and losing some energy to the surface. Mixed modes such as Tapping for topography measurements and non-contact for other force measurements, i.e., magnetic field, are commonly used (see, e.g., U.S. Pat. No. 5,308,974 by Elings et al.).
Hence, the invention will be discussed primarily in terms of TappingMode operation, although it should be understood that the teachings of the invention apply equally well to other oscillating modes.
Referring to FIG. 1, typically for TappingMode operation, a probe 3 having a tip 6 is attached to an oscillator 2 which can drive the probe 3 appropriately, usually at or near the probe's resonant frequency. An electronic signal is applied from an AC signal source 1, under control of an AFM control/computer 7, to the oscillator 2 to drive the probe tip 6 to oscillate at a free oscillation amplitude A.sub.o. A.sub.o can be varied over a broad range, i.e. from hundreds of nanometers to the nanometer scale, the latter is typically used for non-contact force measurements. Practically, for light interaction with a sample surface during imaging, A.sub.o should be as small as possible, but large enough to prevent the tip from sticking to the sample surface 5 due to capillary and/or other adhesive forces. The probe 3 can also be driven toward and away from the sample 5 using a suitable actuator 8 controlled by the computer 7. As shown in FIG. 2, when the actuator 8 is energized to move the probe 3 near the surface and the tip 6 taps the surface of sample 5 at the bottom of each oscillation cycle, the probe tip oscillation amplitude decreases from A.sub.o to A.sub.s. The probe motion could also be controlled magnetically as described in U.S. Pat. No. 5,670,712 by Cleveland et al.
A preset value of A.sub.s is typically used as a setpoint for the vertical feedback servo function for the actuator 8, so the Tapping AFM scans at a predetermined decrease from the free amplitude A.sub.o. Since the probe tip 6 only touches the sample 5 for a short interval at the bottom of each cycle, lateral forces during scanning are virtually eliminated, thereby overcoming the major limitation of contact mode AFMs. For the majority of samples, A.sub.s can be significantly smaller than A.sub.o, often as much as 50% smaller, with no significant sample or tip damage. TappingMode operation is therefore usually immune to small system variations. TappingMode operation is capable of providing stable operation with good image quality and has become the most commercially successful AFM mode.
For some samples however, such as very soft biological samples or large polysilicon grains used in integrated circuit production, even with the lateral forces eliminated, the energy lost to the Tapping action must be extremely small or the tip 6 and/or the sample 5 experience unacceptable wear. Similarly, it is important to limit the force exerted on soft samples to avoid elastic deformation of the sample so that a topographical image accurately represents true sample topography. For these samples, it is preferable that a "light Tapping" mode be employed, where A, is very slightly different than A.sub.o. As may be 95% or even a greater percentage of A.sub.o for "light Tapping", because the tip must continue to tap the sample surface but must remain within a small percentage of A.sub.o. Light Tapping mode, therefore, requires extremely stable oscillation parameters or feedback control to prevent deterioration of the tip-sample interaction.
Holding oscillation parameters (such as amplitude, frequency, and phase) sufficiently stable to allow for light Tapping is difficult to achieve. Even if the electronic drive signal and oscillator outputs are perfectly stable, the actual free oscillation amplitude may vary more than acceptable for light Tapping. Particularly when a new probe is installed in an AFM, the free oscillation may take as much as an hour to stabilize within the 3-5% oscillation amplitude difference range necessary for light Tapping. This may be due to mechanical relaxation of the probe lever itself, settling of the probe-oscillator interface, drift in the probe clamping force, or some other mechanism that is difficult to correct. Thus, when light Tapping parameters are the criteria being used for scanning during settling, there are potential problems; the free oscillation A.sub.o may increase during measurement which will result in increased Tapping forces, or A.sub.o may decrease during measurement which will cause the feedback loop for controlling the actuator 8 to allow the tip 6 to lose intermittent contact with the sample surface, neither of which is acceptable. When the oscillation finally stabilizes, the light Tapping achieves high resolution imaging due to a minimal tip-sample contact area. Tip lifetime is also extended, even on the hardest samples, like ceramics or silicon wafers. However, imaging during the settling time typically results in too many bad scans or excessive tip and/or sample wear. This is a problem because there is often insufficient time to wait for the tip to settle.
Because it is important that the free oscillation parameters remain extremely stable during surface measurement, it is also desirable to isolate the effect of tip interaction with the sample surface from the background effects of the probe's proximity to the surface of the sample. Such background effects may not be present, but often are. One such background effect is discussed by Serry et al. in "Air-Damping of Resonant AFM Micro-Cantilevers in the Presence of a Nearby Surface" and is known as "squeeze film damping." Squeeze film damping may occur if air is located between the probe and the sample surface, in which case the air generally will be compressed by the oscillation of the probe such that the probe's resonant frequency will be reduced or other parameters changed. Similarly, if the probe and sample were electrostatically, magnetically or otherwise affecting each other during probe oscillation, such interplay could affect the oscillation parameters such that the free oscillation changes with proximity to the sample, Thus, a variety of effects may cause the oscillation parameters to shift from the desired preset conditions, which can detract from light Tapping operation.